Cutting Concrete: How a Pro Breaks Through

The first piece of advice is: Sub it out.

While some people might have you believe that diamond blades will make short work of concrete and rebar, nobody can debate that the best way to make a jog or opening in a concrete wall is to form it in the first place. But even veterans make mistakes. Most common are the need to widen rough openings; cut for doors, windows, and access holes; shape forgotten beam pockets, and notch garage foundations at door openings to allow the slab to pour through. Remodeling and additions have their own set of conditions -- often limiting access to the cuts. What's the best way to deal with this?

The first piece of advice is: Sub it out. Specialty contractors can make your life a lot easier, and save you time and money doing it. These guys are set up to handle all size cutting jobs, but even the small cuts can be big trouble. This isn't something to hand off to a helper with an old circ saw and a dust mask. You may never see him again.

"I can't tell you how many times we've had to rescue general contractors who've started cuts they can't finish," says Peter Zoni who owns Concrete Services of Cape Cod, in Orleans, Mass. "They'll spend the money to rent equipment and spend time trying to break through and then call us to finish up." The equipment is key. "We started out using cut-off saws" Zoni says. "We'd score both sides of a wall and then use a 90-lb. jackhammer suspended on ropes to break through." Now Zoni's company uses a track-mounted diamond wall-saw made by Dimas, which can cut up to 18-inches deep, with stepped-up blade diameters. The larger blades alone can cost $1,600. These saws can make it through an 8-inch wall in three passes (depending on rebar position), leaving a smooth finished cut.

Aluminum tracks mounted to the wall guide the big circ saw along the cut. The saw rides each track, cutting deeper on each pass. Water that keeps the blade cool, also helps control dust -- which, given the dangers of silica -- is an important consideration. Even with this ultimate setup it can still take four or five hours to make two vertical door cuts.

Concrete Services also uses a lighter weight hydraulic handheld tool for wall cuts, where precision isn't as important. The Partner Ring Saw looks like a cut-off saw, but it uses an eccentric drive to cut deeper than a cut-off saw with the same diameter blade. "We can cut through a 10-inch thick wall with a 14-inch blade," says Zoni. "You can only cut 5-1/2 inches with a 14-inch blade on a cut-off saw." A 2 x 4 shot into the concrete wall guides the smaller saw along the cut. While the track saw will stay within 1/16-inch accuracy, you can maintain 1/8 to 1/4-inch with the Ring Saw -- with practice. "Dennis Smith, my cut-man, likes the precision of the track saw better," Zoni says, "but I like the Ring Saw." Zoni's crew can cut two door openings a day -- at an average cost of about $500 per opening (which doesn't include removal of the cutouts).

The company also uses a water-cooled, plunge-cutting, diamond chainsaw made by ICS to complete clean corner cuts, beam pockets, and hard-to-reach cuts. "We don't use this as much as we thought we would," say Zoni. "While they're versatile tools, the original blades got gummed-up with slurry, and we still have four unused ones." The company has since come out with new blades that clean out the slurry, so if you buy one of these tools, go for the new blades.

Zoni's crews also use smaller hand tools for a variety of applications. For example, if clean corners are required at an opening, they'll use a 4-inch diameter electric cutoff tool to score the corners, and then finish by chipping the rest out with a small pneumatic chisel. That's because the big circular blade on the wall saw can't cut through without overcutting the corners. And when they're ready to break through they use a combination of small impact hammers, pry bars, and spring bars to persuade the cutout to leave home.

Safety is a big factor in this trade. Noise levels for hammers and saws are extremely high, dust containment is critical, eye protection is a must, and tipping out a 3,000-lb. door cutout can be tricky. So even if you do rent equipment and try to do your own cuts -- take care to prevent injuries.

And if you rent -- here's what you're liable to get. "Unless you also rent a large compressor you're going to end up with an electric jackhammer, that's all size and weight -- but no hitting power," Zoni says. Either way, you won't get the precision cuts. If you rent a saw, it's liable to be a drycut saw that'll create clouds of silica dust. And then there's the blade-wear charge. "It may only cost you $75 to rent the saw for a day, but you might pay $280 when they measure the blade to see how much you used." All in all, it sounds like my old motto still holds true. "When in doubt, sub it out."